A French cab or hackney coach. So called from the Hotel de St. Fiacre, Paris, where the first station of these coaches was established by M. Sauvage, about 1650.
According to Alban Butler, Fiacre was the son of an Irish king, born in 600, to whose tomb pilgrimages were made in the month of August. His day is August 30th. (Lives of the Saints, vol. ii. p. 379.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894